1. Field of the Invention
The present invention relates to a separator, i.e. a Bipolar Plate separator, which is a member of a fuel cell, and a fuel cell using it.
2. Related Art
There are several types of fuel cells, depending on the type of an electrolyte. A phosphoric acid-type fuel cell is of a type using phosphoric acid infiltrated in a support, and is operated at 150 to 220° C. A molten carbonate-type fuel cell is made using a mixture of lithium carbonate and potassium carbonate molded in an electrolyte support, and is operated at 600 to 700° C. A solid oxide-type fuel cell uses stabilized zirconia having an oxygen ion-conducting property as an electrolyte, and is operated at 700 to 1,000° C. In both of the fuel cells, hydrogen, a reformed gas, hydrocarbons or the like is used as fuel, and air is used as an oxidizing gas.
Among various fuel cells, a polymer electrolyte fuel cell (PEFC) and a direct methanol-type fuel cell (DMFC) have a main feature in that a carbon electrode having a catalyst such as platinum carried thereon is bonded to opposite surfaces of a membrane-shaped solid electrolyte formed of a polymer. This assembly is called MEA (a membrane electrode assembly which is also referred to as a membrane/electrode integrated structure). The polymer electrolyte fuel cell assumes a structure called as a separator, in which MEA is clamped between a pair of plates in which channels for a fuel gas (a hydrogen-containing gas) and an oxidizing gas (oxygen or air).
Herein, fuel and an active substance as an oxidizing agent are called collectively a reaction fluid. When each of the fuel and the oxidizing agent is a gas, it called particularly a reaction gas. The following is the description taking, as an example, a case where the fuel and the fluid as the oxidizing agent are gasses.
Usually, MEA and a separator are clamped through a porous carbon sheet. This porous carbon sheet is called a gas diffusion layer and has a function to supply the reaction gas to an electrode with a good efficiency and uniformly. An assembly comprising the MEA, the separator and the gas diffusion layer combined to one another into a set is referred to as a single cell. The fuel cell stack is a structure made by laminating a plurality of simplex cells one on another. The separator has a role to supply the reaction gas to the electrode with a good efficiency, and a power can be taken out by supplying the reaction gas to the fuel cell and applying an appropriate load. This is accompanied by the generation of a heat such as a reaction heat and a joule heat. To remove this heat, usually, the fuel cell is provided with a separator for passing cooling water to a portion of the above-described separator.
The separator has a role to transmit a power to an adjacent cell, while suppressing the loss of energy and hence, is usually formed of a carbon-based conductive material and has through-channels formed therein for passing the reaction gas and a cooling medium. It is also investigated that a thin metal plate is used as a separator material in addition to the carbon-based material. A separator made of a metal has merits that the separator is inexpensive in material cost is low; the separator is easy to form by stamping; and the separator can be made compactly and with a reduced weight, because a thin plate can be used.
In a case of a separator having channels and formed by pressing of a thin metal plate, the processability or workability is restricted to a limit of the processability of a metal material and for this reason, it is difficult to form a channel having a desired depth and a desired width by processing. This causes evil influences that the uniformity of the flowing of the reaction gas is not obtained and that a sufficient area of contact with the electrode cannot be provided, and as a result, it is difficult to provide a desired power-generating performance. Even if the formation of a desired groove by processing is possible, a warpage or a strain may be generated in the separator produced after the processing, and a required finishing accuracy may be not provided in some cases, thereby bringing about the leakage of the reaction gas and an increase in contact resistance.
Another drawback of the separator formed by the pressing of the metal is a problem that each of apexes produced after the formation of the grooves has a curvature, and in a case of an integrated MEA comprising a gas diffusion layer and MEA formed integrally with each other, an area of contact with the integrated MEA is smaller and as a result, the resistance is increased, and a good power-generation performance cannot be obtained. To solve this problem, it is a conventional practice to remove the apex portions having the curvature to flatten the separator (for example, see JP-A-2003-173791).
It is also a conventional practice to form a single separator by a thin metal plate and a carbon paper cut into a channel-defining configuration without carrying out the pressing (for example, see JP-A-2000-123850 and JP-A-2000-294257). The separator of this structure can be produced at a reduced cost. In addition, the channels are created by cutting the carbon paper and hence, the finishing accuracy is higher. Further, the surface of contact of the gas diffusion layer is flat and hence, there is not a problem as arisen in the case of the separator produced by the pressing of the metal.
Besides, in general, the fuel cell generates heat with the generation of a power. Usually, it is required in the fuel cell that cooling water is passed to a cooling cell mounted within the cell to remove a heat and is subjected to the heat exchange. It has been also designed that an oxidizing gas itself is used as a cooling medium in some of molten carbonate-type fuel cells. In the solid polymer electrolyte fuel cell, however, it is not that the cooling by the gas medium is carried out in the majority of cases, for a reason that the temperature of the cell is lower, for a reason that an auxiliary power as small as possible is demanded from the viewpoint of increasing the power-generating efficiency, and for another reason. If the cooling efficiency of the fuel cell is improved, the need for use of a cooling cell, a cooling water pump, a heat exchanger and the like is eliminated and consequently, the simplification of the system is realized.
The separator formed from the thin metal plate and the carbon paper cut into the channel-defining configuration as the gas channel member has many advantages. However, the carbon paper defining the channels is divided into a large number of pieces and hence, a plurality of channel members subdivided in accordance with an increase in number of channels are required. As a result, the following problem is arisen: The number of parts for constituting the cell is increased, and these parts are bound together by a conductive material, resulting in an increase in number of manufacturing steps. In the inventions described in JP-A-2000-123850 and JP-A-2000-294257, it is also a subject to be solved that the prevention of the metal corrosion occurring on the surface of the metallic separator which is in contact with the channel portions is not taken into consideration. In an environment of the cell, the metallic portion is corroded, or a non-conductive passivation film is grown on the metallic portion. As a result, the deterioration of the fuel cell might occur due to an increase in contact resistance, the pollution of the electrodes and the electrolyte membrane by a corrosion product and the like. In addition, no attention is especially paid to the cooling of the body of the fuel cell in all of the above-described patent documents.
It is an object of the present invention to provide a separator which can be produced using a reduced number of parts as compared with the prior art using a carbon paper cut into a channel-defining configuration and which has a higher cooling effect, and a fuel cell including such separator.